a) Field of the Invention
The present invention relates to a semiconductor device and its manufacture method, and more particularly to a semiconductor device having a polycrystalline layer and its manufacture method.
b) Description of the Related Art
Polysilicon layers are used as various elements of various types of semiconductor devices. Following methods are known as manufacture methods for polysilicon layers.
(1) After an amorphous silicon layer is formed, this layer is subjected to a heat treatment for about 50 hours at a temperature of about 600.degree. C. to polycrystallize it. With this method, crystal growth nuclei are formed in the amorphous silicon layer at an earlier stage of the heat treatment, and crystals are grown from these growth nuclei to polycrystallize the amorphous silicon layer.
(2) After an amorphous silicon layer is formed, an energy beam such as a laser beam is applied to this layer to melt it and thereafter cool it. During the cooling process, the molten silicon layer is crystallized and a polysilicon layer is formed.
(3) A silicon layer is formed on a base substrate through chemical vapor deposition or physical vapor deposition at a temperature of about 600.degree. C. Since the substrate is maintained at a sufficiently high temperature, a grown silicon layer becomes a polysilicon layer.
A method of forming various types of semiconductor devices by forming a silicon layer on a glass substrate has been adopted for liquid crystal display devices and solar cells. A polycrystallized silicon layer can improve the performance of a semiconductor device more than a silicon layer of an amorphous phase. Generally, a glass substrate cannot resist a temperature higher than 600.degree. C., and deformation or the like occurs at such a high temperature.
The above-described method (1) requires a heat treatment temperature of about 600.degree. C. A number of stacking faults and twin crystals are formed in a polysilicon layer grown on a glass substrate and it is difficult to form a polysilicon layer having good crystallinity.
The above-described method (3) requires a temperature of 600.degree. C. or higher when the film is formed. Even if a polysilicon layer is grown on a glass substrate, crystallinity is not sufficient similar to the method (1) and it is difficult to form crystals providing a high mobility of carriers.
A remaining possibility of growing a polysilicon layer on a glass substrate is the above-described method (2). A polysilicon layer formed by utilizing laser annealing has high quality. However, the conditions of forming high quality polysilicon are restricted and very narrow. A method capable of stably forming a high quality polysilicon layer has been desired.